EPO 0 385 298 A2 shows such a pump but is has the problem that its operation is unavoidably associated with vibrations of variable magnitude, based on imbalance of the impeller and the electric motor as well as the magnetic rotating field that is produced. The vibrations are transmitted to the apparatus connected to the pump, and cause a high-frequency operating noise.
In order to reduce this noise, it has been proposed in DE-A1 41 07 049.6 to support the electric motor by means of two elastic motor mounts between the electric motor and the surrounding housing. The pump impeller in this citation has several parts, which must be counterbalanced.
U.S. Pat. No. 2,857,849 discloses a pump in which several pumpimpellers are attached to an electric motor shaft, by a screw connection. Since the threads of the screw connection must be sufficiently rigid, it is concluded that the impellers are made of metal. These are heavy and have large inherent imbalances.
An electric air pump of this type is disclosed in DE 196 11 512.4 and its equivalent U.S. Pat. No. 5,797,727, in which the pump mechanism has one or more pump impellers, injection-molded of plastic, whose impeller hubs are press-fit onto an electric motor shaft and bear against a stop ring in the direction they are press-fit on the shaft and the hubs are secured by a metal locking ring press-fit on the shaft at the end opposite the collar. The contacting surfaces of the impellers and the locking ring that contact one another are formed with an interlocking tooth and groove arrangement.
This construction provides a low noise operation of the pump since lighter pump impellers can be used. In addition, a very simple construction for the electrical air pump is obtained with this design, which can be produced at relatively low cost.
However, it has been found that the plastic impeller hubs press-fit on the motor shaft can nevertheless rotate on the motor shaft which leads to an axial displacement of the impeller and the locking ring, whereby the function and performance of the electric air pump are adversely affected and impact noise on the stop ring may occur.
Experiments were conducted using adhesives between the impeller hubs and the motor shaft to prevent slippage, but due to gaps in the adhesive between the hubs and the shaft, the hubs continued to loosen leading to the above disadvantages.